For a high voltage direct current converter, a power semiconductor capable of controlling a turn-off is used for bidirectional conversion of AC voltage and DC current. Since the maximum voltage that a power semiconductor can withstand is limited, a plurality of semiconductor modules having power semiconductor circuits have to be connected in series for high voltage processing. In order to configure a power semiconductor circuit, various semiconductor modules may be connected to each other.
As is known, a known modular multilevel converter (MMC) includes a plurality of sub-modules in which such a power semiconductor circuit forms two output terminals and the plurality of sub-modules are connected in series. Such a sub-module may include, for example, an energy storage unit and a power semiconductor circuit formed of a plurality of power semiconductor switches and free wheel diodes.
FIG. 1 illustrates such a known MMC converter. A corresponding converter is configured to have one or more phase modules 1, in which a plurality of sub-modules 10 are connected in series. As a load connecting terminal, AC voltage side terminals L1, L2, and L3 may be connected to a three-phase load, for example, a three-phase power system.
FIG. 2 illustrates an example of equivalent circuits of the sub-modules 10. Each of the sub-modules 10 may include an energy storage unit 11, power semiconductor switches 12 and 13, which are connected to the energy storage unit 11 in parallel and capable of controlling a turn-off, and at least one power semiconductor circuit 16 including free wheel diodes 14 and 15. Each of the sub-modules 10 may be implemented to have various configurations featuring different arrangements of the energy storage unit 11 and the at least one power semiconductor circuit 16. Each of the sub-modules 10 includes first and second connecting terminals X1 and X2.
In addition, when a failure occurs in a specific sub-module 10 in the MMC converter, the sub-module 10 in which the failure occurred is short-circuited in order to prevent an open circuit of a phase module 1. Due to the short circuit, phase current is bypassed from the failed sub-module 10 to enable the phase module 1 to be normally operated by another normal sub-module 10. As a short circuit for shorting the sub-module 10, for example, a vacuum interrupter tube 100 is provided. The vacuum interrupter tube 100 may be controlled by a control unit (not illustrated) to be shorted within several msec after failure occurs. Accordingly, in normal operation, normal current flows through the power semiconductor circuit 16 of the sub-module 10, but at the time of failure in the specific sub-module 10, the vacuum interrupter tube 100 of the failed sub-module 10 is shorted and the phase current is bypassed through the vacuum interrupter tube 100 to protect the phase module 1.
FIG. 3 is a cross-sectional view of the vacuum interrupter tube 100, and FIG. 4 is a side view of a control device for controlling the operation of the vacuum interrupter tube 100 of FIG. 3. The inside of the vacuum interrupter tube 100 is maintained in a vacuum state by a vacuum sealed container. A fixed contactor 101 is built into a fixed contact bolt 111 and a moveable contactor 102 is built into a movable contact bolt 112. In addition, first and second output terminals X1 and X2 are respectively connected to the fixed contact bolt 111 and the movable contact bolt 112. Accordingly, a short circuit is formed or released by contact or separation of the fixed contactor 101 and the movable contactor 102. Holding power 200 occurs in the movable contact bolt 112 in a vertical direction due to the pressure difference between the inside and the outside of the vacuum interrupter tube 100 in order to enable the movable contactor 102 to move toward the fixed contactor 101. Such holding power 200 is supported by a spring operation of an internal metal bellows 120 and an air pressure difference between the inside and outside of the vacuum interrupter tube 100. Accordingly, power 240 applied in an opposite direction to the holding power 200 is necessary in order to release the short circuit in the vacuum interrupter tube 100. This opposite power 240 is provided by the control device 300.
In an operation process, there is an interval 335 between a core contactor 310 and the soft magnetic core 320 of the control device 300. At the time of normal operation, power is applied to a coil 340 wound around the core 320, the core 320 operates as an electromagnet to attract the core contactor 310, and the movable contact bolt 112 connected to the core contactor 310 is interlocked to separate the fixed contactor 101 from the movable contactor 102. Accordingly, a gap 150 is created between the fixed contactor 101 and the movable contactor 102 to release the short circuit. At the time of the occurrence of failure, the power supply to the coil 340 is cut off, and the fixed contactor 101 and the movable contactor 102 are brought into contact with each other by the foregoing power 200 to form a short circuit.
However, for the typical control device 300, at the time of normal operation, power is continuously supplied to the coil 340 for releasing the short circuit of the vacuum interrupter tube 100, which causes power loss. In addition, when the converter is in a power failure state, power may not be supplied to the coil 340, and in this case, it is not possible to release the short circuit of the fixed contactor 101 and the moveable contactor 102, and accordingly there is no method for supplying power to the converter through the AC voltage side terminals L1, L2, and L3, or DC voltage side terminals P and N to operate the converter.